Saw blade tensioning arrangement

ABSTRACT

A saw-blade tensioning arrangement for band saw and like machines which includes a hydraulic cylinder (6) having arranged for movement therein a piston (7) which is intended to activated saw-blade tension control means (2). The blade tension is, in this way, dependent on the hydraulic pressure in the cylinder. The arrangement includes means for pneumatically regulating the hydraulic pressure, these means including a pneumatically operated pressure booster (10) and means (19, 20, 35) for activating the pressure booster at the end of a working stroke of the hydraulic piston, therewith to tension the saw blade to the desired working tension, this working tension being determined, inter alia, by the blade width.

The present invention relates to an arrangement which is intended fortensioning the cutting blades of band saws and like machines and whichcomprises a hydraulic piston-cylinder device the piston of which isintended to activate blade tension control means, such that the tensionin the blade is governed by the hydraulic pressure in the cylinder ofthe piston-cylinder device.

In band saws which are equipped with blade tensioning facilities, e.g.the band saw described and illustrated in No. SE-7312757-3 the blade istensioned with the aid of an hydraulically operated piston which isurged upwards by means of pump pressure, this pressure being heldconstant with the aid of a pressure-compensated pump and diversepressure regulators. This in combination with a solid steel pistonresults in a rigid, inelastic tensioning system with no resilience orability to absorb shock.

With the growing demand for sawing machines of higher performancelevels, it has been found desirable to introduce resiliency in bladetension in order, inter alia, to be able to compensate for those changesin blade length which result from heating and cooling of the blade. No.SE-7317517-6 describes a completely gas-sprung system, whichnevertheless is encumbered with certain drawbacks.

The main object of the present invention is to provide a bladetensioning arrangement which incorporates a hydraulic piston-cylinderdevice and with which at least the aforesaid disadvantage inherent withknown hydraulic devices is eliminated.

This object is achieved in accordance with the invention by controllingthe hydraulic pressure pneumatically, thereby introducing resiliency andshock absorbing facility into the system and enabling the blade tensionto be regulated or controlled in a particularly simple manner.

Accordingly, this invention consists in a blade tensioning arrangementof the kind described in the introduction which is characterized in thatthe arrangement includes means for regulating the hydraulic pressurepneumatically; in that the regulating means include a pneumaticallyoperated pressure booster or amplifier; and in that means are providedfor activating the pressure booster at the end of a working stroke ofthe piston, such as to achieve the desired working tension in the sawblade.

The inventive arrangement is thus controlled solely by the pneumaticsincorporated in the arrangement, therewith enabling the blade tension tobe controlled and regulated in a very simple fashion, asbeforementioned. Furthermore, since the hydraulic pump found in priorart blade tensioning systems has been replaced with, inter alia, apneumatically operated pressure booster, it is possible to create adesired degree of resilience or spring in the system. It is essentialthat a facility is provided which will enable the blade tension of aband saw to be regulated readily and simply, since the correct bladetension is directly proportional to the blade width, which changes eachtime the blade is ground. Consequently, a pneumatic pressure regulatoris preferably connected to the pressure booster, so as to enable theblade to be tensioned to a desired working tension and maintained atthis level of tension. The use of such a pneumatic pressure regulatorcan be highly beneficial, among other things because it is simple tocontrol.

In order to achieve the requisite amount of spring, or shock absorbency,in the system, an additional volume of air can be made available betweenthe pressure booster and the pressure regulator. In accordance with apreferred embodiment of the agreement, the hydraulic piston-cylinderdevice has connected thereto a pressure sensor which initiatesactivation of the pneumatic pressure booster when the hydraulic pressurereaches a preset value.

In accordance with a particularly preferred embodiment of the invention,which provides a very compact construction of the whole blade tensioningarrangement, the hydraulic piston is hollow and the pressure booster islocated within the piston and is carried by the piston during pistonmovement. In this case, the outlet opening of the pressure booster ispreferably connected to a leadthrough in the bottom of the piston. Thepiston is preferably fully sealed and contains the requisite amount ofoil for maneuvering the piston and also incorporates the pneumaticconnections required for working air and return air.

Other characteristic features of the invention are set forth in thefollowing claims.

The invention will now be described in more detail with reference toexemplifying embodiments illustrated in the accompanying drawings.

FIG. 1 illustrates schematically a band saw provided with an arrangementaccording to the invention.

FIG. 2 is a sectional view of a hydraulic piston-cylinder device havinga hollow piston and functioning as a saw stand, and also illustrates apneumatic control circuit operative in controlling the band sawillustrated in FIG. 1.

FIGS. 3 and 4 correspond to the illustration of FIG. 2, but illustrateother stages of a working cycle.

FIG. 5 illustrates an alternative embodiment of the pneumatic controlcircuit.

FIG. 6 illustrates another embodiment of the invention applied with ahydraulic piston-cylinder device which is positioned horizontally.

FIG. 1 illustrates a band saw which comprises an upper saw-blade wheel 1which is journalled on a shaft 2, and a lower saw-blade wheel 3 which iscarried by a laterally adjustable mounting block 4. Extending over theblade wheels is an endless saw blade 5, the tension in the blade beingadjusted by changing the distance between the blade wheels. To this endthe mounting block of the band saw has the form of or incorporates ahydraulic piston-cylinder device 6, having a hydraulic piston 7 whichcarries the shaft 2 of the upper blade wheel 1 and which is axiallymovable in the cylinder.

FIG. 2 is a sectional view of the hydraulic piston-cylinder device 6, 7which functions as a chain-saw stand and the piston 7 of which carriesthe shaft 2 of the upper saw-blade wheel. The hydraulic piston 7 ishollow and has arranged therein a pneumatically operated pressurebooster 10 comprising a cylindrical high pressure part 11, into which arod-like plunger 12 is driven with the aid of a further plunger 14which, in turn, is driven into the low-pressure part 13 of the pressurebooster and which has a larger piston-area than the rod-like plunger 12.Arranged in the lower end surface of the cylinder 11 is an opening whichcommunicates directly with a lead-through 15 extending through thebottom surface of the hydaulic piston 7. The reference 16 identifies athroughflow passage in the upper end of the high-pressure cylinder 11.The piston 7, the high-pressure cylinder 11 and the hydraulic cylinder 6are filled with oil 17, the pressure of which is measured by means of amanometer 18. The reference numeral 19 identifies a so-calledpressostat, which is intended to send a signal to a relay unit 20, overa signal conductor 38 when detecting a pre-determined pressure. Further,it gives an alarm signal in case of a pressure drop.

The references 21, 22 and 23 identify three pneumatic connections on thepiston 7, of which connections the connection 21 communicates with aspace 24 formed above the surface of the oil 17 in the piston 7, whereasthe connection 22 communicates, through a pipe 25, with the space abovethe plunger 14 in the low-pressure part of the pressure booster 10. Thepneumatic connection 23 communicates with the lower part of thelow-pressure part 13, through a pipe 26.

The reference 27 identifies a connection to an air pressure circuit ornetwork, where reference 28 identifies a closure valve, reference 29identifies an air filter and reference 30 identifies a non-return valve.Extending from the non-return valve 30 is a first conduit 31 whichextends to the air connection 21 via a three-path valve 32, while asecond conduit 33 is connected to a secondary, ventilated pressureregulator 34, which can be placed in communication with respectivepneumatic connections 22 and 23 via a pressure-controlled solenoid valve35. Reference 36 identifies an operating pressure conduit for the valve35 and the reference 37 identifies a signal conductor extending from therelay unit 20 to the valve 35.

In order to achieve improved resilience and shock absorbency in thesystem, an auxiliary air source 39 can be connected between the valve 35and the pressure regulator 34.

With the band saw in the illustrated operational state it is assumedthat the upper saw-blade wheel shall be raised from a lowered positionto the working position, for example after having changed the saw blade.The upper wheel is raised by switching the valve 32 to the positionshown in the Figure, whereupon compressed air from the network 27 issupplied to the space 24 above the oil 17 in the piston 7. The oil willtherewith flow through the free flow passage 16 in the high-pressurepart of the pressure booster 10 and through the leadthrough 15 to theunder surface of the piston 7, where it exerts a lifting pressure on thepiston which consequently lifts the upper saw-blade wheel to its workingposition.

When the solenoid valve 35 is set to the position illustrated in FIG. 2,the space above the plunger 14 in the low-pressure part of the pressurebooster is ventilated and the plunger 14 is urged to its upper positionby compressed air supplied to the connection 23, such as to uncover thethroughflow passage 16 in the high-pressure part of the pressurebooster.

The pressure booster 10, however, must be activated in order to tensionthe saw blade to the high level of working tension required. Thepressure booster 10 is activated when the hydraulic pressure in thecylinder 6, engendered by the compressed air and measured by means ofthe manometer 18, has reached a value pre-set on the pressostat 19. Whenthis pressure is reached, the pressostat sends a signal along theconductor 38 to the relay unit 20, which in turn sends a switchingsignal to the solenoid valve 35.

When the solenoid valve 35 is switched to the position illustrated inFIG. 3, air under pressure, set by the pressure regulator 34, issupplied to the space above the plunger 14, through the connection 22,at the same time as the space below the undersurface of the piston isventilated through the connection 23. As the plunger 14 is pressed down,the rod-like plunger 12 will close the throughflow passage 16 and extendinto the high-pressure part 11 of the pressure booster 10, therewithincreasing the pressure of the oil in the cylinder 6, which correspondsto the ratio between the cross-sectional areas of the plungers 14 and12. This increased hydraulic pressure results in final tensioning of thesaw blade to a value corresponding to the pressure set on the pressureregulator 34.

As will be understood from the aforegoing, the whole of the tensioningsequence can be effected without using jacks or hydraulic pumps, bycontrolling the hydraulic pressure in the cylinder 6 pneumatically. Thisfacility is highly beneficial, since, inter alia, it enables the use ofrelatively simple components and air control systems. The aforedescribedarrangement is also extremely compact and requires hardly any additionalspace, since the pressure booster can be incorporated in the hollowpiston forming part of the band saw stand. The presence of air on theupper side of the plunger 14 also imparts a degree of resilience, i.e.spring, to the system, which resilience can be enhanced by introducingadditional air into the system from the auxiliary air source 39, asillustrated.

When wishing to relax the saw blade, e.g. when making a blade change,the settings of the solenoid valve 35 and the valve 32 are reversed, asillustrated in FIG. 4. The plunger 14 and the rod-like plunger 12 willthen be lifted, so as to expose the throughflow channel 16 and permitoil to flow up from the cylinder 6 through the high-pressure part of thepressure booster 11. The spaces above the plunger 14 and above the oilin the piston 7 are ventilated through respective valves. The piston 7and the upper saw-blade wheel can then be lowered, wherewith the speedat which the saw-blade wheel is lowered can be controlled by throttlingthe return air in the valve 32.

Instead of utilizing an auxiliary air supply for achieving the desiredamount of spring in the system, the air volume 24 above the oil level inthe piston 7 can be used in accordance with the FIG. 5 illustration,which corresponds to the state illustrated in FIG. 3, wherewithcorresponding components are identified with the same reference numeralsas those used in the preceding Figures. The FIG. 5 embodimentincorporates an additional solenoid valve 39 which enables a switch tobe made between non-reduced pressure and reduced pressure via thepressure regulator 34. Because this enables a direct connection to beused between the valves 32 and 35, the volume of air located above theoil level in the piston 7 in the illustrated high-pressure case will beincorporated in the system and therewith afford the requisiteresiliency.

In order to provide an indication of unacceptable leakage of hydraulicoil from the system, the plunger 14 in the pressure booster may bemagnetic or provided with a magnetic part, such that it is able toinitiate an alarm signal should the plunger sink beneath apre-determined level. This is an essential safety facility. Theaforementioned non-return valve 30 in the air supply system is alsoprovided for safety reasons, in order to prevent accidents should theair supply suddenly cease.

In those cases when the piston according to the invention is notincorporated in the saw stand, or when there is used a recumbenthydraulic cylinder, the alternative embodiment of the inventionillustrated schematically in FIG. 6 can be used. Correspondingcomponents in FIG. 6 are identified with the same reference numerals asthose used in preceding Figures. It will be seen from FIG. 6 that thepressure booster 10 of this embodiment is not incoporated in the piston7 but has the form of a separate component. Since the piston 7 no longerserves as a pressure vessel, it is necessary to provide an outerpressure vessel 40 which contains oil and which can be placed underpressure with the aid of compressed air from the air supply 27, bysetting the valve 32.

It will be understood by those skilled in this art that the method ofoperation of the FIG. 6 embodiment is the same as that of the earlierdescribed embodiment. It will also be understood that an auxiliary airsupply can be provided if desired from the aspect of system resiliency.

The invention has been described in the aforegoing with reference to anumber of preferred embodiments illustrated in the drawing. It will beunderstood, however, that these embodiments may be modified within thescope of the following claims, particularly with regard to the pneumaticpart of the system. For example, the pressure regulator can be adjustedmore or less automatically in dependence on the blade tension required,which in turn is determined, for instance, by the prevailing bladewidth.

I claim:
 1. A saw-blade tensioning arrangement for band saws and likemachines, including a hydraulic cylinder (6) having movably arrangedtherein a piston (7) which is intended to activate saw-blade tensionregulating means (2), so that the blade tension becomes dependent on thehydraulic pressure in the cylinder (6), characterized by means forpneumatically controlling the hydraulic pressure, these means includinga pneumatically operated pressure booster (10), and means (19, 20, 35)for activating the pressure booster (10) at the end of a working strokeof the hydraulic piston (7), so as to tension the saw blade (5) to adesired working tension.
 2. An arrangement according to claim 1,characterized by a pneumatic pressure regulator (34) which is connectedto the pressure booster (10) for adjusting the saw blade (5) to adesired working tension and for maintaining the saw blade at the settension level.
 3. An arrangement according to claim 2, characterized inthat an auxiliary air volume (39) is connected between the pressurebooster (10) and the pressure regulator (34) in order to enhance theresiliency of the tensioning arrangement.
 4. An arrangement according toany of claims 1-3, characterized by a pressure sensor (19) which isconnected to the hydraulic cylinder (6) and which is intended toinitiate activation of the pneumatic pressure booster (10) when thehydraulic pressure has reached a pre-set value.
 5. An arrangementaccording to claim 1, characterized in that the hydraulic piston (7) ishollow, and in that the pressure booster (10) is incorporated in thepiston and is carried thereby during piston movement.
 6. An arrangementaccording to claim 5, characterized in that the outlet opening of thepressure booster (10) is connected to a leadthrough (15) in the bottomof the piston (7).
 7. An arrangement according to claim 5, characterizedin that the piston (7) is sealed and contains the amount of oil (17)required for the operation of the piston and that the piston (7) isprovided with connections (21, 22, 23) for working air and return air.8. An arrangement according to claim 7, characterized by means (32) forsupplying working air to an air space (24) above the oil (17) in thepiston (7), means (16, 15) for permitting oil to be pressed out on theunderside of the piston (7) through the high-pressure part (11) of theinactive pressure booster (10), such as to cause the piston (7) toexecute a low-pressure stroke controlled by said oil of a pressurecorresponding to the air pressure of said air space (24).
 9. Anarrangement according to claim 7, characterized by means (34, 35) forsupplying working air of pre-set pressure to the pressure booster (10),and that the pressure booster is provided with means (14, 12, 15) bymeans of which oil at elevated pressure is pressed out on the undersideof the piston (7), therewith to cause the piston to execute itshigh-pressure stroke.
 10. An arrangement according to claim 1,characterized by monitoring means for establishing when a plunger (12)incorporated in the pressure booster (10) sinks beneath a pre-set level.